1. Field of the Invention
The invention relates to a polishing pad which is useful for creating a smooth, ultra-flat surface on such items as glass, semiconductors, dielectric/metal composites, and integrated circuits. More particularly, the invention relates to a molded polishing pad having a window which facilitates inspection of a workpiece and determination of a polishing endpoint by optical means.
2. Discussion of Related Art
Semiconductor wafers having integrated circuits fabricated thereon must be polished to provide a very smooth and flat wafer surface which in some cases may vary from a given plane by as little as a fraction of a micron. Such polishing is usually accomplished in a chemical-mechanical polishing (CMP) operation which utilizes a chemically active slurry that is buffed against the wafer surface by a polishing pad.
One problem associated with chemical-mechanical polishing is determining when the wafer has been polished to the desired degree of flatness. Conventional methods for determining a polishing endpoint require that the polishing operation be stopped and that the wafer be removed from the polishing apparatus so that dimensional characteristics can be determined. Stopping the operation slows down wafer production. Further, if a critical wafer dimension is found to below a prescribed minimum, the wafer may be unusable, thereby leading to higher scrap rates and production costs.
In-process methods for determining polishing endpoint have also been developed. One such method utilizes laser interferometry wherein light generated by a laser is used to measure a wafer dimension. See, for example, U.S. Pat. No. 5,413,941.
Polishing pads have been developed with features that facilitate the determination of wafer dimensional characteristics by optical methods. U.S. Pat. No. 5,605,760 discloses a polishing pad wherein at least a portion of the pad is transparent to laser light over a range of wavelengths. In one embodiment, the entire polishing pad is a transparent sheet which may be made out of any solid uniform polymer including polyurethanes, acrylics, polycarbonates, nylons and polyesters. In another embodiment, the polishing pad includes a transparent window piece in an otherwise opaque pad. The window piece may be a rod or plug of transparent polymer material in a molded polishing pad. The rod or plug may be insert molded within the polishing pad, or may be installed into a cutout in the polishing pad after the molding operation.
U.S. Pat. No. 5,893,796 also discloses a polishing pad having a window piece provided by a transparent plug.
The plug may be preformed as a solid insert that is molded into the pad. Alternatively, the plug may be formed by pouring liquid polyurethane into a hole that has been cut into the polishing pad, and the polyurethane may be cured to form a transparent plug within the pad.
The prior art polishing pads having transparent window pieces have a number of disadvantages. Manufacturing steps are required to either install the window piece into a hole in the pad, or into the mold cavity in which the pad is produced. In some cases a hole to receive the window piece must be cut into the pad. Leakage of slurry between the pad and the window piece may be a problem. Also, since the window material is different than the pad material, the window and the pad may wear at different rates. This may lead to cracking or tearing of the pad around the window during polishing. There is a need for a polishing pad having a transparent window that overcomes these problems.